The invention relates generally to circuits for driving large inductive loads. More specifically, the invention relates to a current driver capable of producing fast charges and discharges of an inductor.
X-ray scanning is a popular method for use in a variety of everyday applications, including medical diagnostics, industrial imaging, and security systems. Commercially available x-ray sources typically utilize conventional thermionic emitters, which are helical coils made of conductive wire and operated at high temperatures. Each thermionic emitter is configured to emit a beam of electrons to a single focal spot on a target. To obtain a total current of 10 to 20 mA with an electron beam size of 10 mm2, helical coils formed of a metallic wire having a work function of 4.5 eV must be heated to about 2600K. Tungsten wire is a popular choice for forming the helical coil due to its robust nature.
Alternative devices are also used for providing an x-ray source for an x-ray scanning system. For example, such devices are described in co-owned, co-pending U.S. application Ser. Nos. 11/048,158 and 11/048,159, both filed Feb. 1, 2005. Common to the different x-ray sources is that these sources represent large inductive loads that are operated by a current. The current for the x-ray sources or inductors is driven by circuits that are meant to charge and discharge the inductor quickly while still providing accurate current levels. However, due in part to the number of switches these driving circuits typically require, these driving circuits can be expensive and often experience high losses. Furthermore, as the system operates in a charging/discharging mode and a steady state mode that each require different voltage levels, the number of power sources necessary for the system increases the expense of the system and limits the transition time between the operating modes. Additionally, during the steady state operation of the inductive load, high ripple can occur due in part to the voltage levels.
It would therefore be desirable to have a deflection coil current driving circuit having a minimum number of switches and power sources to increase the transition time, reduce ripple, and reduce cost. Additionally, to assure accurate current levels through the inductive load, a pulse width Modulation scheme for the analog circuit is also desirable.